Many portable electronic appliances such as cell phones, music players, notebook computers, cameras, and other such devices have USB ports for connection to host computers with which the appliances exchange data such as address lists, digital photographs, music, videos, text, and so on. A USB port has a +5 volt power line, a D+ data line, a D− data line, and a ground line. The host computer provides DC electrical power to the power line, and the host computer communicates with the appliance through the data lines.
In addition to communicating through the data lines, the appliance can draw electrical power from the host computer through the power and ground lines. This power may be used to operate the appliance, to charge its battery, or for other purposes. The amount of power that can be drawn from the host computer is limited in some cases to about one-half watt (100 milliamps at 5 volts) and in other cases to about 2.5 watts (500 milliamps at 5 volts). At these relatively low power levels, charging the battery in the appliance can take several hours.
To avoid the need for connecting to a host computer just to charge the battery, and to provide faster charging times, dedicated charging power supplies have been developed. These power supplies are typically smaller than host computers, in some cases less than one cubic inch. They connect to the appliance through the USB port and can provide significantly more charging current than host computers, in some cases up to 10 watts (2 amps at 5 volts).